1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for an automotive vehicle. More specifically, the invention relates to an anti-skid brake control system which has variable brake control characteristics depending upon the vehicle speed. Further, particularly, the invention relates to an automotive anti-skid brake control system which has a vehicle speed dependent slippage threshold feature.
2. Description of the Background Art
The Japanese Patent First Publication (Tokkai) Showa 56-79043 discloses an anti-skid brake control system.
In general, wheel slippage is taken as a criterion for reducing hydraulic brake pressure in a wheel cylinder for preventing vehicular wheels from skidding. It is well known that an optimum braking performance can be obtained by maintaining the wheel slippage at about 15% to 25%. Therefore, it is a conventional technique to set a wheel slippage threshold at the aforementioned range to reduce the brake pressure when the wheel slippage exceeds the set wheel slippage threshold.
Such a conventional process is generally successful in preventing the vehicular wheel from skidding. However, it has been found that optimum wheel slippage at which the optimum braking performance can be obtained is variable depending upon the vehicle speed. That is, when the vehicle speed is relatively low, such as below 40 km/h, the optimum braking performance can be obtained at a wheel slippage of about 25%. On the other hand, when the vehicular speed is relatively high, such as higher than 40 km/h, the wheel slippage at which the optimum braking performance can be obtained, is lowered. Such variation of the wheel slippage at which the optimum braking performance can be obtained, is considered to be caused by hysteresis of the elastic components, such as rubber bushing, damping characteristics of dampers in the suspension system and so forth.